In this study, a two-stage system converting organic solid waste (food waste + sewage sludge) to H-2 and CH4 was operated. In the first stage of dark fermentative hydrogen production (DFHP), a recently proposed method that does not require external inoculum, was applied. In the second stage, anaerobic sequencing batch reactor (ASBR) and an up-flow anaerobic sludge blanket reactor (UASBr) were followed to treat H-2 fermenter effluent. (H-2 + CH4-ASBR) system showed better performance in terms of total biogas conversion (78.6%), while higher biogas production rate (2.03 L H-2/L-system/d, 1.96 L CH4/L-system/d) was achieved in (H-2 + CH4-UASBr) system. To reduce the alkali addition requirement in DFHP process, CH4 fermenter effluent was tested as a diluting water. Both the ASBR and UASBr effluent was effective to keep the pH above 6 without CH4 production. In case of using ASBR effluent, H-2 production dropped by 15%, but alkali addition requirement was reduced by 50%. (C) 2013 Elsevier Ltd. All rights reserved.